A squelch circuit is used in high frequency devices to filter out noise signals, and prevent the hiss sound when no signals are present. With the squelch circuit, the input signal must exceed a certain level to be regarded as a meaningful signal for acceptance. Conventional squelch circuits usually compare the input signal, amplify to full swing, and directly determine if the input signal is meaningful by checking the output. FIG. 1 shows the conventional squelch process. The squelch input operation identifies the positive and negative voltages of input signals, and the full swing squelch operation compares the signals and amplifies them to full swing.
To amplify the signals to full swing, the squelch circuit needs to generate a large electrical current, especially when operating at high frequency. This, in turn, needs a large circuit area. FIG. 2 shows a conventional squelch circuit, wherein D and DB represents the positive and negative voltages of the input signal, respectively. The gain of this circuit depends on the input current and the dimension of the transistors. For full swing, the circuit requires a large area. At a lower operating frequency, there are approaches to obtaining high gain without sacrificing limited circuitry area. However, these approaches are not suitable for high frequency operation as the gain decays at high frequencies.
To amplify the signals to full swing, the squelch circuit needs to generate a large electrical current, especially when operating at high frequency. This, in turn, needs a large circuit area. FIG. 2 shows a conventional squelch circuit, wherein D and DB represent the positive and negative voltages of the input signal, respectively. The gain of this circuit depends on the input current and the dimension of the transistors. For full swing, the circuit requires a large area. At a lower operating frequency, there are approaches to obtaining high gain without sacrificing limited circuitry area. However, these approaches are not suitable for high frequency operation as the gain decays at high frequencies. As a result, there is a need in having a squelch circuit that uses a pre-amplifier without a high gain to amplify the input signal, i.e., a low swing pre-amplifier that does not provide full swing amplification.